1. Field of the Invention
The present invention relates to a variable capacity device having quantum-wave interference layers with an nin or pip junction structure. The present invention also relates to a device with an nn.sup.- p, np.sup.- p, or nip junction structure.
2. Description of the Related Art
A voltage-variable capacity device has been known to have a pn junction structure, whose p-layer having a high impurity concentration and n-layer having a low impurity concentration are jointed and form a depletion layer at a boundary region between the p-layer and the n-layer as a capacity device. When a reverse bias voltage is applied to the pn junction of the variable capacity device, a width of the depletion layer is extended and a capacity of the variable capacity device is reduced. Thus the capacity of the device varies according to a value of applied reverse bias voltage.
With respect to the voltage-variable capacity device, it is required to enlarge a variation rate of a capacity value toward an applied voltage. To enlarge the variation rate, forming space distribution in an impurity concentration has been suggested. And to enlarge the voltage variation rate, i.e., voltage-sensitivity, the impurity concentration is required to have a non-linear distribution with respect to a depth.
To obtain the non-linear distribution of the impurity concentration, injecting ions whose acceleration voltage is varied, or a modulation doping in the process of a crystal growth is suggested. But because of a thermal diffusion of impurities, it is difficult to obtain the non-linear distribution as accurate as designed. Thus improvement of the voltage-variation rate of capacity is limited.